When designing electronic systems, signals are often required which have certain prescribed waveforms, such as sinusoidal, square, triangular, etc. Electronic systems in which such signals are required include: computer and control systems in which clock pulses are used for timing purposes; communication systems in which a variety of waveforms are used as information carriers; and test and measurement systems in which a variety of waveforms are used for testing and characterizing electronic devices and circuits. A large number of well-known circuits are used for generating such signals, and are generally known as oscillators.
In many applications, it is desirable to provide a signal with a frequency that is independent of variations in supply voltage. Conventional supply voltage insensitive oscillators typically employ one or more of a variety of well-known voltage regulation circuits in order to provide the oscillator with a regulated voltage, which is relatively independent of the supply voltage. Alternatively, supply-insensitive current sources are used to drive the oscillator with a relatively constant current, independent of variations in the supply voltage.
One particularly important application of supply voltage insensitive oscillators is for self-refresh circuits integrated within dynamic random access memories (DRAMs). Self-refresh circuits are used to refresh the memory contents of a DRAM when in a powered-down state or otherwise not regularly accessed and refreshed by external circuitry, such as a DRAM controller. The self-refresh circuit integrated within the DRAM includes an oscillator which provides a clock-like signal to control periodic refreshing of the data stored in the DRAM. Proper timing of refresh operations is crucial to accurate retention of stored data. It is therefore desirable that the frequency produced by the self-refresh circuit be relatively independent of variations in the supply voltage applied to the DRAM. However, conventional supply voltage insensitive oscillators have properties which are disadvantageous in DRAM applications--e.g, occupying relatively large portions of semiconductor area. and requiring relatively large currents.